This invention relates to an improved process and reaction system for continuous production of liquid ammonium polyphosphate compositions. More particularly, this invention is directed to a novel reactor configuration and improved reaction technique for conversion of wet-process phosphoric acid having a low polyphosphate content into an ammonium polyphosphate product having a high porportion of the total P.sub.2 O.sub.5 in the polyphosphate form by reaction with anhydrous ammonia in a continuous process employing a staged reaction zone in which the heat of the ammoniation reaction is utilized to preheat the reactants while minimizing scale build up on the reactor walls and reversion of polyphosphate to orthophosphate.
Ammonium polyphosphate based compositions are materials of commerce, having utility as fire retardants and fertilizers. In the fertilizer end use, where high analysis liquid or solid products are desired, it is advantageous and even essential that a major amount of the P.sub.2 O.sub.5 be present as pyrophosphate and other longer chain phosphates for optimum handling properties in field applications. With high analysis ammonium polyphosphate fluids, the presence of this high proportion of condensed phosphates provides a stable liquid phase and enhances the sequestration capability of the composition when metal impurities and/or other additives such as micronutrients are present (as in blended formulations). With solid ammonium polyphosphate products, high polyphosphate contents provide improved control of crystal size which is inherently more variable and difficult to control in solid formulations containing large amounts of monoammonium phosphate and diammonium phosphate.
As a result of the foregoing and other incentives, extensive research has been carried out on techniques and process modifications for maximizing the polyphosphate content of the product obtained from the reaction between ammonia and phosphoric acid. Since wet-process phosphoric acid is an inexpensive and widely available source of raw material for this process, much of the published literature in this area of technology is directed to processes for increasing the polyphosphate content of the wet-process acid either before or during reaction with ammonia. This wet-process phosphoric acid is typically employed in a crude form and, as a result, it contains a variety of metal (iron, aluminum, etc.) impurities which can form troublesome deposits in processing equipment (pipeline reactors) and/or in the resultant ammonium phosphate solutions unless a high proportion of the P.sub.2 O.sub.5 present is in the polyphosphate form.
Early attempts to maximize the polyphosphate content of ammonium phosphate based compositions involved adding super phosphoric acid or ammonium salts of super phosphoric acid to the wet process phosphoric acid ammoniation reaction system e.g. see U.S. Pat. Nos. 3,015,540 and 3,507,614 to Striplin. Another approach taken early on was to concentrate or remove water from merchant strength wet-process phosphoric acid i.e., orthophosphoric acid, via a separate evaporation step to provide a concentrated or super wet-process acid which could be ammoniated to form high analysis fertilizer base solutions in which deposit-forming metal contaminants are substantially or totally sequestered e.g. see U.S. Pat. Nos. 3,044,851 and 3,192,013 to Young. Subsequently, in U.S. Pat. No. 3,382,059 to Getsinger, it was disclosed that the autogeneous heat of reaction between anhydrous ammonia and wet-process phosphoric acid could be employed to condense the acid and form ammonium polyphosphate solutions directly without a separate heating and concentrating step.
Following the Getsinger Patent disclosure of in situ dehydration or condensation of wet-process phosphoric acid using the autogeneous heat of the ammoniation reaction, a variety of processes and/or process modifications have appeared in the literature using this concept to advantage. Many of these processes employ a wet-process super phosphoric acid feed stock i.e., acid containing at least some superphosphoric acid, and optimize one or more of the reaction parameters-i.e. temperature, residence time, reactant mixing, weight ratios and the like--to maximize the polyphosphate content of the reaction product. U.S. Pat. No. 3,464,808 to Kearns, U.S. Pat. No. 3,503,706 to Legal, U.S. Pat. No. 3,695,835 to Kearns, U.S. Pat. No. 3,775,534 to Merline, U.S. Pat. No. 3,730,700 to Groenveld and U.S. Pat. No. 3,950,495 to Ries are examples of teachings in this general area of technology. In these and other disclosures it is common practice to carry out the ammoniation reaction in a pipeline or tubular reactor and, in some cases, a staged reaction system is usefully employed. U.S. Pat. No. 3,733,191 to Meline et al and Defensive Publication T 909,016 to Mann disclose staged reaction systems which include pipeline reactors. In some cases one or both of the reactants (anhydrous ammonia and phosphoric acid) are preheated and, on occassion, the exothermic heat of the ammoniation reaction is used to advantage for this function. In this regard, U.S. Pat. No. 3,939,255 to Moore et al is of particular interest in that it teaches a continuous ammoniation process in which wet-process phosphoric acid containing at least 55% P.sub.2 O.sub.5 and anhydrous ammonia are both at least partially preheated by indirect heat exchange with the reaction mass. While the Moore et al process apparently affords an ammonium polyphosphate melt having a sufficiently high proportion of the P.sub.2 O.sub.5 present in the polyphosphate form that solids do not form in the reaction system or on conversion of the melt to high analysis solutions, the process is rather complex and expensive requiring three separate heat exchangers (two steam heated exchangers for initial heating of the reactants and one phosphoric acid-reaction product exchanger) in addition to a jacketed reaction zone for heat exchange between partially heated ammonia and the reaction mass.
From the foregoing it can be seen that continuous effort has been expended in the area of technology to improve and optimize the production of high analysis ammonium polyphosphate fluids from wet-process phosphoric acid. However, despite this effort and the multiplicity of process modifications which have resulted therefrom, a variety of process problems e.g. foaming, corrosion, solids deposition etc., and complexities still remain, depending on the process selected, which provide incentive for further research in this technical area.